6 research outputs found
Ocean-bottom seismographs based on broadband MET sensors: architecture and deployment case study in the Arctic
The Arctic seas are now of particular interest due to their prospects in terms of hydrocarbon extraction, development of marine transport routes, etc. Thus, various geohazards, including those related to seismicity, require detailed studies, especially by instrumental methods. This paper is devoted to the ocean-bottom seismographs (OBS) based on broadband molecular–electronic transfer (MET) sensors and a deployment case study in the Laptev Sea. The purpose of the study is to introduce the architecture of several modifications of OBS and to demonstrate their applicability in solving different tasks in the framework of seismic hazard assessment for the Arctic seas. To do this, we used the first results of several pilot deployments of the OBS developed by Shirshov Institute of Oceanology of the Russian Academy of Sciences (IO RAS) and IP Ilyinskiy A.D. in the Laptev Sea that took place in 2018–2020. We highlighted various seismological applications of OBS based on broadband MET sensors CME-4311 (60 s) and CME-4111 (120 s), including the analysis of ambient seismic noise, registering the signals of large remote earthquakes and weak local microearthquakes, and the instrumental approach of the site response assessment. The main characteristics of the broadband MET sensors and OBS architectures turned out to be suitable for obtaining high-quality OBS records under the Arctic conditions to solve seismological problems. In addition, the obtained case study results showed the prospects in a broader context, such as the possible influence of the seismotectonic factor on the bottom-up thawing of subsea permafrost and massive methane release, probably from decaying hydrates and deep geological sources. The described OBS will be actively used in further Arctic expeditions
Numerical Modeling of Nonlinear Response of Seafloor Porous Saturated Soil Deposits to SH-Wave Propagation
Numerical modeling of seismic response of soil deposits is usually conducted as part of seismic hazard assessment, preceding facility construction in any tectonically active regions, including offshore sites. A significant feature of subsea soils is their porous and water-saturated structure. Thus, the purpose of the present study is to introduce a procedure for modeling nonlinear behavior of porous, moist soils during SH-wave propagation, to verify it and compare response for synthetic soil profiles with porous medium parameters specific for low moisture onshore and high moisture offshore sites with cohesive and non-cohesive soils. The well-known and approved NERA code was used as a basis and improved to incorporate the Biot and Gassman equations for elastic waves propagation in a fluid-saturated porous solid. The applicability of the presented approach was substantiated for integration into other well-known algorithms. Obtained results showed good agreement between the simulated by different methods and observed spectra. The modeling also showed that the response of cohesive and non-cohesive soils with moisture specific both for onshore and offshore sites is explained by effects of resonances and effect of seismic amplitude saturation, which, in turn, depend on the corresponding value of the layer thickness and S-wave impedance for porous saturated soil layer. The proposed scheme could have significant practical usage for studying the effect of porous medium parameters on the seismic response of the moist soil deposits
Peculiarities of the HVSR Method Application to Seismic Records Obtained by Ocean-Bottom Seismographs in the Arctic
The application of the horizontal-to-vertical spectral ratio (HVSR) modeling and inversion techniques is becoming more and more widespread for assessing the seismic response and velocity model of soil deposits due to their effectiveness, environmental friendliness, relative simplicity and low cost. Nevertheless, a number of issues related to the use of these techniques in difficult natural conditions, such as in the shelf areas of the Arctic seas, where the critical structures are also designed, remain poorly understood. In this paper, we describe the features of applying the HVSR modeling and inversion techniques to seismic records obtained by ocean-bottom seismographs (OBS) on the outer shelf of the Laptev Sea. This region is characterized by high seismotectonic activity, as well as sparse submarine permafrost distribution and the massive release of bubble methane from bottom sediments. The seismic stations were installed for one year and their period of operation included periods of time when the sea was covered with ice and when the sea was ice-free. The results of processing of the recorded ambient seismic noise, as well as the wave recorder data and ERA5 and EUMETSAT reanalysis data, showed a strong dependence of seafloor seismic noise on the presence of sea ice cover, as well as weather conditions, wind speed in particular. Wind-generated gravity waves, as well as infragravity waves, are responsible for the increase in the level of ambient seismic noise. The high-frequency range of 5 Hz and above is strongly affected by the coupling effect, which in turn also depends on wind-generated gravity waves and infragravity waves. The described seafloor seismic noise features must be taken into account during HVSR modeling and interpretation. The obtained HVSR curves plotted from the records of one of the OBSs revealed a resonant peak corresponding to 3 Hz, while the curves plotted from the records of another OBS did not show clear resonance peaks in the representative frequency range. Since both OBSs were located in the area of sparse distribution of submarine permafrost, the presence of a resonance peak may be an indicator of the presence of a contrasting boundary of the upper permafrost surface under the location of the OBS. The absence of a clear resonant peak in the HVSR curve may indicate that the permafrost boundary is either absent at this site or its depth is beyond the values corresponding to representative seismic sensor frequency band. Thus, HVSR modeling and inversion techniques can be effective for studying the position of submarine permafrost
MatNERApor—A Matlab Package for Numerical Modeling of Nonlinear Response of Porous Saturated Soil Deposits to P- and SH-Waves Propagation
The paper is devoted to the problem of numerical modeling of earthquake response of porous saturated soil deposits to seismic waves propagation. Site-specific earthquake response analysis is a necessary and important component of seismic hazard assessment. Accounting for the complex structure of porous saturated soils, i.e., the content in them, in addition to the solid matrix, pore water, gas mixture and ice, is especially important for the water areas in the zones of continuous or sparse permafrost, as well as the massive release of bubble gas from bottom sediments. The purpose of this study is to introduce an algorithm and its Matlab implementation for numerical modeling of the nonlinear response of porous saturated soil deposits to vertical P- and SH-waves propagation. The presented MatNERApor package consists of a set of Matlab scripts and functions. The package was tested and verified using the records of vertical seismic arrays of the Kik-net network. In addition, the records of local earthquakes obtained by ocean bottom seismographs in the Laptev Sea in 2019–2020 were used to demonstrate the effect of the water layer above the seabed sites on the reduction of vertical motions spectra. The results of the calculations showed good agreement with the data obtained from real seismic records, which justifies the correctness of the theoretical basis of the presented algorithm and its software implementation
A Complex of Marine Geophysical Methods for Studying Gas Emission Process on the Arctic Shelf
The Russian sector of the arctic shelf is the longest in the world. Quite a lot of places of massive discharge of bubble methane from the seabed into the water column and further into the atmosphere were found there. This natural phenomenon requires an extensive complex of geological, biological, geophysical, and chemical studies. This article is devoted to aspects of the use of a complex of marine geophysical equipment applied in the Russian sector of the arctic shelf for the detection and study of areas of the water and sedimentary strata with increased saturation with natural gases, as well as a description of some of the results obtained. This complex contains a single-beam scientific high-frequency echo sounder and multibeam system, a sub-bottom profiler, ocean-bottom seismographs, and equipment for continuous seismoacoustic profiling and electrical exploration. The experience of using the above equipment and the examples of the results obtained in the Laptev Sea have shown that these marine geophysical methods are effective and of particular importance for solving most problems related to the detection, mapping, quantification, and monitoring of underwater gas release from the bottom sediments of the shelf zone of the arctic seas, as well as the study of upper and deeper geological roots of gas emission and their relationship with tectonic processes. Geophysical surveys have a significant performance advantage compared to any contact methods. The large-scale application of a wide range of marine geophysical methods is essential for a comprehensive study of the geohazards of vast shelf zones, which have significant potential for economic use